Fabaceae

The Fabaceae family, known as legumes, are one of the most important plant families in both ecological and economic terms. Legumes help increase soil nitrogen and provide rich sources of vegetable protein for humans, livestock, and wild animals.

Structure

The flowers of the legume family are diverse but uniformly bilaterally symmetric. Indeed, fifty-million-year-old fossil legume flowers provide the first instance of bilaterally symmetrical flowers in the fossil record of flowering plants. Most legumes have five petals and five sepals , with the sepals commonly fused at least at the base. The five petals commonly occur as one large upper petal, two lower petals that clasp the ovary and stamens, and two lateral petals that often act as a platform for a landing bee (or other insect). There are deviations to this pattern, including a common one where petals are all alike and arranged in a radially symmetric fashion even though the ovary or stamens always retain the bilateral symmetry.

The fruits of the legume family are also varied. Most commonly, one fruit (the pod) is produced per flower. The pod has two valves, each bearing a seed along the upper margin. The two valves together form the single compartment of most pods. When the fruit matures, the two valves often twist apart forcefully, catapulting the enclosed seeds. However, pods of many legume species remain intact and disperse with the mature seed. The seed then germinates from within the fallen pod. The pod may not have a single compartment, but rather can be transversely segmented such that each seed is enclosed in its own compartment. Pods can be small and contain one seed or linear to circular and contain dozens of seeds.

The distinctive aspect to the vegetative morphology of legumes is the compound leaf, mostly pinnately compound, but also palmately compound. Less common are simple legume leaves. Legume leaves are deciduous during the dry season in the tropics or during the winter in temperate regions. Deciduous leaves are considered an adaptation to habitats with seasonally varying moisture availability, which is especially the case in many tropical regions.

The morphology of the legume fruit is dependent upon how the pod disperses. Water-dispersed fruits have a thick buoyant outer covering that contains many air cells (e.g., Andira ). Wind-dispersed fruits often bear wings of varying sizes and shapes (e.g., Dalbergia ). Pods carried away on the fur of passing animals bear different kinds of hairs, ornaments, or glands to make them sticky (Desmodium, for example). The structure attaching the seed to the inside of the fruit is sometimes fleshy. This fleshy structure (the aril, as in Pithecellobium ) is firmly attached to the seed and is often colorful and sweet and serves to disperse the seed (e.g., by birds). Legume seeds are highly variable but always include a preformed embryo with two large cotyledons. The cotyledons are rich in nitrogen compounds (such as alkaloids or nonprotein amino acids), most of which are toxic to animals. Selection during domestication has resulted in the loss of toxins while retaining the nitrogen-rich compounds. Legumes cultivated for their edible nitrogen rich seeds (e.g., beans, peas, and lentils) are referred to as pulses. Soybeans form an important part of the diet in Asia and are used as a protein-rich livestock feed in many countries.

Distribution, Symbiosis, and Economic Importance

The Fabaceae predominate in most vegetation types of the world. Legume species are most abundant in seasonally dry, tropical forests, but they also abound in high deserts to lowland rain forests. They are notably uncommon in high alpine sites with abundant summer rains and in the southern beech forests of the Southern Hemisphere. Woody legume species are mostly confined to tropical and subtropical habitats, but herbaceous species occur from the tropics to cold temperate regions. Thus, many legume species are available for use in reforestation or revegetation projects worldwide. Their efficient ability to acquire nitrogen, phosphorus, and other essential nutrients, whether through nodulation or association with mycorrhizae (symbiotic root fungi), allows them to easily establish on abused lands where soils are eroded, leached, or acidic. Legumes can be found as pioneer species

or as major constituents in secondary vegetation. Disturbed lands characterized by high erosion, leaching of nutrients, or accumulation of salts can become readily inhabited by legume species. Notably, an association of mycorrhizae and legumes is just as important in this regard as is the legume-rhizobia association. This is especially true when soils are acidic. The nitrogen-rich metabolism of legumes is a desired property for revegetation because by killing the legume (naturally or intentionally), nitrogen is released for use by non-leguminous associates. Green manuring a worn-out field with clover, for instance, can help restore soil fertility.

A remarkable feature of the Fabaceae is the high nitrogen metabolism that occurs in all members of this family. Legumes are so nitrogen-demanding that they have evolved several mechanisms to efficiently scavenge organic and inorganic nitrogen from the soil. One of these is the formation of root nodules upon infection by rhizobia. Rhizobia is the collective name for bacteria that infect legume roots and fix atmospheric nitrogen, which then becomes available to the legume plant. Many legumes are susceptible to being infected by rhizobia, whereupon the rhizobia are localized by the legume plant in root, rarely stem, nodules.

Legumes and rhizobia have a symbiotic relationship, which benefits the legume but is not necessary for its growth. Legumes appear to acquire nitrogen by other means than nodulation first, and an influx of soil nitrogen, natural or otherwise, can cause a nodulating legume to cease its association with rhizobia.

Few if any other plant families have as many species that are so economically important worldwide as the Fabaceae. This is especially true for both industrial and nonindustrial economic species. Peas, beans, and lentils (the pulses) are probably the most important industrial species, but luxury timbers (e.g., rosewoods) are also important here. The most economically important nonindustrial species include multipurpose tree species, such as Leucaena leucocaphala or Gliricidia sepium, which are cultivated for shade in crop fields (as in coffee plantations), local timber, livestock forage, and cover for reforestation projects. Hundreds of herbaceous and woody legume species are cultivated regionally or globally as ornamentals, livestock forage, green manure , or as sources of gums, medicinal products, or secondary metabolites.

see also Biogeochemical Cycles; Kudzu; Nitrogen Fixation; Plant Community Processes; Savanna; Soybean; Symbiosis.

Matt Lavin